The present invention relates to the field of ultrasonic sensors, and particularly to an ultrasonic sensor for determining objects in air or other gaseous media and which contains a first ultrasonic transducer having a piezoelectric body as the transmitter.
Sensors are devices which are designed so that they can detect or quantitatively determine physical parameters such as pressure, temperature, position or velocity over a measuring range. Due to the growth of electronic industrial process control systems, the demand for more complex sensors at relatively low prices has increased. a suitable sensor is, for instance, an ultrasonic proximity switch which can detect the presence of objects or persons. Other types of sensors, such as inductive or capacitive proximity switches, pose problems in switching ranges of 100 cm. Furthermore, ultrasonic proximity switches can also determine the presence of small objects with higher security against interference. Practically all materials are suitable for this type of object determination.
For such ultrasonic echo sounding systems, ultrasonic transducers are provided which generate a ray of sound with a particularly small aperture angle. Interfering reflections of the ultrasonic energy can thereby be avoided so that particularly high resolution can be achieved and objects located close to each other can be distinguished. In an ultrasonic transducer, the dimensions are essentially given by the operating frequency provided and the material of the transducer. The dimensions of the ultrasonic transducer also determine the size of the sound radiating area of the transducer, the aperture angle of the sound ray generated being determined essentially by the size of the sound-radiating surface. For an operating frequency of about 49 kHz and lead zirconate-titanate PZT as material for the transducer, an aperture angle of about 10.degree. to 12.degree. is obtained, for instance.
An ultrasonic transducer is known which contains a peizoelectric body, a .lambda./4 matching layer and a loading ring, where .lambda. is the wavelength. On one end face of the piezoelectric body, the .lambda./4 matching layer is arranged, the diameter of which is substantially larger than the diameter of the piezoelectric body. The surface region of the .lambda./4 matching layer which extends beyond the edge of the piezoelectric body is provided with the loading ring. See, e.g., German DE-AS No. 24 41 492. It is achieved by the provision of a loading ring that a large area, the dimensions of which are substantially larger than the end face of the piezoelectric body, is excited to in-phase vibrations.
Further known is an ultrasonic proximity switch which operates as a distance sensor without touch or contact. The heart of this proximity switch is a piezoceramic ultrasonic transducer which operates in air or other gases. In the mode of operation as a proximity switch, the object to be determined is used as an ultrasonic reflector, where the usable detection range is between 20 and 100 cm and the unusable near range is between 0 and 20 cm. The objects to be determined may be solid, liquid or in powder form with a plane, smooth, polished or mat surface. The material on these surfaces may, in addition, be transparent or be of any color. See, e.g., H. CH. Muenzing "Range Sensor for Large Switching Distances", ETZ, vol. 103 (1982) No. 10, pages 518-519. This known ultrasonic proximity switch only detects objects which are suitable for reflection and the reflection surface of which is arranged perpendicularly to the axis of the sound lobe within the 50% width. Deviations of no more than .+-.3.degree. from the perpendicular to the axis of the sound lobe are permissible.